Hard contact lenses now on market may be broadly classified into two groups. Those classified in one group are hitherto known hard contact lenses prepared from methyl methacrylate (MMA) as their principal material, which have no oxygen permeability. Hard contact lenses classified in the other group are recently developed ones prepared from silicone resin, fluoride resin or cellulosic material as their principal material, which have good oxygen permeability. Although such oxygen permeable hard contact lenses are recognized as more advantageous in their fitness on the eye than hitherto known ones, it is known that the oxygen permeable hard contact lenses are liable to be soiled, which is one of their defects. Being less susceptible to soiling, even the hitherto known hard contact lenses also become soiled caused from the deposit of eye discharges, proteinous substances, saccharides, mucosaccharides or inorganic substances, if such hard contact lenses are worn on the eye for a long period of time. Such is known as an inevitable defect of the hard contact lenses. It is known that, if the soiled contact lens is worn on the eye for a long period of time, the soil not only irritates the eye but also becomes the cause of growth of bacteria or fungi adhered on the contact lens utilizing these soils as their nutrient source to cause serious injuries to the eye. Thus, the removal of the soils adhered on the contact lens is an important problem to be solved. To remove such soil and to prevent such injuries, therefore, a variety of cleaning solutions have been proposed and employed. These cleaning solutions, however, have disadvantages in that they give harmful effect to the lens and/or an unclear end point of the cleaning is observed. Further, many of such cleaning solutions are not potent enough to clean a heavily soiled hard contact lens and particularly, as far as proteinous soils are concerned, there has not been known any effective method for cleaning hard contact lenses.